(i) Field of the Invention
The present invention relates to a surface acoustic wave filter (hereinbelow, referred to as an SAW filter) for use in a radio frequency unit of mobile communication equipment, particularly, a portable telephone and relates to a branching filter comprising the filter. This application is a counterpart application of Japanese Application Serial Number 160636/1999, filed Jun. 8, 1999, the subject matter of which is incorporated herein by reference.
(ii) Description of the Related Art
Hitherto, as an art regarding an SAW filter, for example, there is one disclosed in xe2x80x9cStructure of transmitting SAW filter for branching filterxe2x80x9d of Japanese Patent Application Laid-Open No. 10-256869 (1998). FIG. 8 shows the structure of a branching filter comprising conventional SAW filters. As shown in FIG. 8, the branching filter comprising the SAW filters is constructed as described hereinbelow. An antenna-side matching circuit 7 is coupled to an antenna terminal 1 and an earth terminal 2. Between a transmitting terminal 3, an earth terminal 4, and the antenna-side matching circuit 7, a transmission-side branching circuit 8, a transmission-side SAW resonator type filter 9, and a transmission-side matching circuit 10 are coupled. On the other hand, between a receiving terminal 5, an earth terminal 6, and the antenna-side matching circuit 7, a reception-side branching circuit 11, a reception-side SAW resonator type filter 12, and a reception-side matching circuit 13 are coupled.
FIG. 9 shows the construction of a radio frequency unit (RF unit) including a branching filter constructed by using general SAW resonators. As shown in FIG. 9, a branching filter 15 is coupled to an antenna 14. An amplifier 16 and a reception-side SAW filter 17 are coupled to the reception side of the branching filter 15. A power amplifier 18 and a transmission-side SAW filter 19 are coupled to the transmission side of the branching filter 15.
In this instance, FIG. 10 shows the structure of the reception-side SAW resonator type filter 12 as an example. The reception-side SAW filter 12 shown in FIG. 10 has a ladder-shaped six-stage structure using SAW resonators. Between an input terminal 21 and an output terminal 22, a first serial-arm SAW resonator 23, a second serial-arm SAW resonator 24, and a third serial-arm SAW resonator 25 are coupled. Between the input terminal 21 and the ground potential, a first parallel-arm SAW resonator 26 is coupled. Between the ground potential and node between the first and second serial-arm SAW resonators 23 and 24, a second parallel-arm SAW resonator 27 is coupled. Between the ground potential and node between the second and third serial-arm SAW resonators 24 and 25, a third parallel-arm SAW resonator 28 is coupled. Between the output terminal 22 and the ground potential, a fourth parallel-arm SAW resonator 29 is coupled.
As kinds of the branching filters, two kinds of branching filters, namely, a branching filter comprising dielectric filters for use in a communication terminal such as a portable telephone and an SAW branching filter for use in a card terminal can be mainly mentioned so far. Since a demand for miniaturization of a communication terminal device itself is increased in association with the spread of communication terminals such as portable telephones in recent years, a demand for miniaturization of high frequency parts built in the communication terminal device is also increased. Consequently, the SAW branching filter is used in place of the dielectric filer also in the communication terminal such as a portable telephone. Requested standards for the SAW branching filter as a device come to be equivalent to those for the branching filter comprising the dielectric filters.
When the SAW branching filter is used in the communication terminal such as a portable telephone, as compared with the case where it is used in the card terminal, the electric power resistance of comb teeth electrodes of the SAW filter in the SAW branching filter is regarded as important. It is strongly desired that the SAW branching filter satisfies standards regarding an amount of attenuation in the communication terminal such as a portable telephone. Further, the above-mentioned electric power resistance of the SAW branching filter is obtained, and on the other hand, it is desired that an amount of attenuation in the vicinity of a frequency pass band regarding the reception-side SAW filter, namely, in a frequency pass band regarding the transmission-side SAW filter is obtained as much as possible while realizing the miniaturization of the chip size of the SAW filter. Also in the SAW branching filter using a cascade-connecting dual-mode SAW filter, while the electric power resistance of comb teeth electrodes is obtained, and on the other hand, it is desired that an amount of attenuation in the vicinity of the frequency pass band regarding the reception-side SAW filter, namely, in the frequency pass band regarding the transmission-side SAW filter is obtained as much as possible while realizing the miniaturization of the chip size of the SAW filter.
According to the present invention, it is an object to provide an SAW filer in which while realizing the miniaturization of the chip size of a SAW filter, the electric power resistance of comb teeth electrodes in the SAW filter can be obtained, and an amount of attenuation in the vicinity of a frequency pass band regarding one SAW filter, namely, in a frequency pass band regarding the invention other SAW filter can be obtained as much as possible and provide a branching filter utilizing it.
To accomplish the above object, according to the present invention, there is provided an SAW filter comprising: a first serial-arm SAW resonator coupled to an input terminal; and a first dual-mode SAW resonator coupled between the first serial-arm SAW resonator and an output terminal, wherein the first dual-mode SAW resonator includes first to third comb-shaped interdigital transducers, the second comb-shaped interdigital transducer is arranged between the first and third interdigital transducers and also coupled between the first serial-arm SAW resonator and the ground potential, and each of the first and third comb-shaped interdigital transducers is coupled between the ground potential and the output terminal.
To accomplish the above object, according to the present invention, there is provided an SAW branching filter comprising: a reception-side surface acoustic wave filter which is coupled between an antenna terminal and an amplifier; and a transmission-side surface acoustic wave filter which is coupled between the antenna terminal and a power amplifier, wherein an input terminal of the reception-side surface acoustic wave filter is coupled to the antenna terminal and an output terminal (of the reception-side surface acoustic wave filter) is coupled to the amplifier, the reception-side surface acoustic wave filter has a first serial-arm SAW resonator coupled to the input terminal and a first dual-mode SAW resonator coupled between the first serial-arm SAW resonator and the output terminal, the first dual-mode SAW resonator includes first to third comb-shaped interdigital transducers, the second comb-shaped interdigital transducer is arranged between the first and third comb-shaped interdigital transducers and also coupled between the first serial-arm SAW resonator and the ground potential, and each of the first and third comb-shaped interdigital transducers is coupled between the ground potential and the output terminal.